EXPERIMENT
5
A Cycle of Copper Reactions
PURPOSE
To observe a sequence of reactions of copper that form a
cycle, along with the color and physical property changes
that indicate those reactions. To gain skill in recording
observations and interpreting them in terms of chemical
equations. To use a simple classification scheme
for grouping chemical reactions by reaction type. To
practice quantitative laboratory techniques by determining the percentage of the initial sample of copper that is
recovered.
PRE-LAB PREPARATION
To a beginning student of chemistry, one of the most fascinating aspects of the laboratory is the dazzling array of
sights, sounds, odors, and textures that are encountered
there. Among other things, we believe that this experiment will provide an interesting aesthetic experience.
You will be asked to carry out a series of reactions
involving the element copper and to carefully observe
and record your observations. The sequence begins
and ends with copper metal, so it is called a cycle of
copper reactions. Because no copper is added or removed
between the initial and final steps, and because each
reaction goes to completion, you should be able to
quantitatively recover all of the copper you started
with if you are careful and skillful. This diagram
shows in an abbreviated form the reactions of the cycle
of copper:
HNO
NaOH
∫∫∫B
Cu ∫∫∫B3 Cu(NO3)2 ∫∫∫B Cu(OH)2
(1)
(2)
Zn, HCl
(5)
H SO
CuSO4 b∫2∫∫4 CuO
(4)
Like any good chemist, you will probably be curious to
know the identity of each reaction product and the stoichiometry of the chemical reactions for each step of the
cycle. Here they are, numbered to correspond to the steps
shown in the chemical equation.1
8HNO3(aq) 3Cu(s) O2(g) B
3Cu(NO3)2(aq) 4H2O(l) 2NO2(g)
(1)
Cu(NO3)2(aq) 2NaOH(aq) B
Cu(OH)2(s) 2NaNO3(aq)
(2)
Cu(OH)2(s) B CuO(s) H2O(l)
(3)
CuO(s) H2SO4(aq) B CuSO4(aq) H2O(l)
(4)
CuSO4(aq) Zn(s) B ZnSO4(aq) Cu(s)
(5)
These equations summarize the results of a large number of experiments, but it’s easy to lose sight of this if you
just look at equations written on paper. You can easily be
overwhelmed by the vast amount of information found
in this lab manual and in chemistry textbooks. It is in fact
a formidable task to attempt to learn or memorize isolated bits of information that are not reinforced by your
personal experience. This is one reason why it is important to have a laboratory experience. Chemistry is preeminently an experimental science. As you perform this—
and any other—experiment, watch closely and record
what you see. Each observation should be a little hook in
your mind on which you can hang a more abstract bit of
information, such as the chemical formula for the compound you are observing.
heat
(3)
1
Labels specify the states of the reactants and products: (s) means a solid, (l)
means a liquid, (g) means a gas, and (aq) means an aqueous (water) solution.
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72
A CYCLE OF COPPER REACTIONS
It is also easier to remember information that is organized by some conceptual framework. Observations and
facts that have not been assimilated into some coherent
scheme of interpretation are relatively useless. It would be
like memorizing the daily weather reports when you have
no knowledge of or interest in meteorology.
Chemists look for relationships, trends, or patterns of
regularity when they organize their observations of
chemical reactions. The periodic table, which groups the
elements into chemical families, is a product of this kind
of thinking. Each element bears a strong resemblance to
other members of the same chemical family but also has
its own unique identity and chemistry.
In a similar fashion, it is useful to classify reactions
into different types. Because no one scheme is able to
accommodate all known reactions, several different
kinds of classification schemes exist. A simple classification scheme we will use at the beginning is one based
on ideas of precipitation (ion combination), acid-base
(proton transfer), and redox (electron transfer). Here we
present an outline and some examples of this kind of
classification:
A SIMPLE SCHEME FOR CLASSIFYING
CHEMICAL REACTIONS
1. PRECIPITATION REACTIONS (THE COMBINATION OF POSITIVELY CHARGED IONS WITH
NEGATIVELY CHARGED IONS TO FORM AN
INSOLUBLE NEUTRAL COMPOUND THAT PRECIPITATES FROM SOLUTION). If we add a solution
of sodium chloride (NaCl) to a solution containing silver
nitrate (AgNO3), an insoluble white solid forms. The
solid, called a precipitate, is silver chloride (AgCl), and we
may write the following chemical equation to describe
the reaction, using symbols to represent the substances in
solution.
Na(aq) Cl(aq) Ag(aq) NO3(aq) B
AgCl(s) Na(aq) NO3(aq)
solid AgCl precipitate is easily separated from the solution containing the soluble sodium nitrate salt. If we
desired, we could also recover the sodium nitrate by evaporating the water from the solution.
2. ACID-BASE (PROTON TRANSFER REACTIONS).
An acid is a substance that reacts with water to form
hydronium ions (H3O) by transfering a proton to a
water molecule.
(a strong acid) HCl(g) H2O B H3O(aq) Cl(aq)
(a weak acid)
CH3COOH(aq) H2O j H3O(aq)
CH3COO(aq)
HCl is a strong acid, completely dissociated in aqueous
solution, while acetic acid (CH3COOH) is a weak acid
that only partially dissociates into hydronium ion and
acetate ion.
A base is a substance that forms hydroxide ions when
dissolved in water:
(a strong base) NaOH(s) B Na(aq) OH(aq)
(a weak base)
NH3(g) H2O j NH4(aq) OH(aq)
NaOH is a strong base, completely dissociated in aqueous
solution, while NH3 is a weak base, only partially dissociating into ammonium ion and hydroxide ion.
Acids react with bases to form salts and (usually) water.
Both are neutral compounds, being neither strongly acidic
nor strongly basic. So acid-base reactions are also called
neutralization reactions. Two examples follow:
(a strong acid a strong base)
H3O(aq) Cl(aq) Na(aq) OH(aq) B
2 H2O Na(aq) Cl(aq)
(the net ionic equation)
H3O(aq) OH(aq) B 2 H2O
First we eliminate the ions called spectator ions, which
appear on both sides of the equation but do not participate in the precipitation reaction. What is left is the net
ionic equation:
(a weak acid a weak base)
CH3COOH(aq) NH3(aq) B NH4(aq) CH3COO(aq)
Ag(aq) Cl(aq) B AgCl(s)
Note that in these two examples of acid-base reactions a
proton is transferred from the acid to the base.
The net ionic equation concisely summarizes the net
result of mixing the two solutions: the formation of an
insoluble precipitate when a positively charged silver ion
is combined with a negatively charged chloride ion. The
3. REDOX REACTIONS (ELECTRON TRANSFER
REACTIONS). Oxidation-reduction reactions, called
redox reactions, are reactions that involve the shift or
transfer of electrons from one kind of atom to another. In
A CYCLE OF COPPER REACTIONS
some reactions, the transfer is obvious, as in this reaction:
Mg(s) 2H3O(aq) 2Cl(aq) B
H2(g) Mg2(aq) 2Cl(aq)
Here, each magnesium atom is giving up two electrons to
two hydrogen ions, forming magnesium ion and hydrogen gas.
Sometimes, the transfer of electrons between atoms is
less obvious, as in the following reaction:
2SO2(g) O2(g) B 2SO3(g)
Here, the reactants and products are all gases, and no ions
are formed. In classifying this reaction as an oxidationreduction reaction, we use the concept of assigning oxidation numbers (also called oxidation states) to each
atom in the compound. A simple set of rules defines the
procedure for assigning the oxidation number. For a simple binary compound (a compound composed of two
different elements), we imagine that all of the electrons in
the chemical bonds are assigned to the atoms with the
greatest affinity for electrons. The ability of an atom to
attract electrons to itself is called electronegativity, and the
atoms having the greatest electronegativity are found in
the upper right hand corner of the periodic table, with
fluorine having the most.
In sulfur dioxide, SO2, and sulfur trioxide, SO3, we
imagine that all of the electrons in the S—O bonds are
assigned to the O atoms, giving each oxygen atom a full
valence shell; this formally gives each oxygen atom a net
charge of 2. So if oxygen is assigned oxidation number
2, the sulfur in SO2 must have oxidation number 4
and the oxidation number of S in SO3 would be 6, since
the sum of the oxidation numbers on all the atoms must
add up to the net charge on the molecule (zero, in this
case). The oxidation number of atoms in their elemental
form is always assigned zero. This seems reasonable for
O2 because the oxygen atoms are equivalent so that there
would be no tendency for one oxygen atom to transfer
electrons to its partner in the O2 molecule.
Once we have assigned oxidation numbers to each element in the chemical reaction, we will see that in this particular reaction the oxidation number of the sulfur atoms
increases from 4 to 6 while the oxidation number of
the oxygen atoms decreases from zero (in O2) to 2 (in
SO3). From this viewpoint, the change in oxidation number is formally equivalent to transferring electrons from
sulfur to oxygen. We say that the sulfur atoms have been
oxidized (because their oxidation number increases),
while the oxygen atoms in O2 have been reduced (because
the oxidation number of oxygen atoms decreases from
zero in O2 to 2 in SO3).
73
We must be careful to note, however, that the oxidation
numbers we assign do not necessarily represent the real distribution of electronic charge in the molecule. By assigning
the oxidation number according to fixed rules, we have
artificially assigned integer changes in oxidation number to
particular atoms (sulfur and oxygen in this case), but the
changes in the electron density on the sulfur and oxygen
atoms may not be as large as implied by the assigned oxidation numbers. Nevertheless, it is reasonable to suppose that
the sulfur atom in SO3 is more positive than the sulfur
atom in SO2 because the added oxygen atom would tend to
draw electrons away from the sulfur atom.
4. DECOMPOSITION REACTIONS (A SUBSTANCE
BREAKING DOWN INTO SIMPLER SUBSTANCES
UNDER THE INFLUENCE OF HEAT). Although
many chemical reactions can be classified into one of the
three reaction types we described earlier, it is possible to
find examples of chemical reactions that do not neatly fit
into this scheme. For example, when calcium carbonate is
heated, it breaks down into simpler substances:
CaCO3(s) B CaO(s) CO2(g)
This reaction might be called a decomposition or dissociation reaction. Any substance heated to a sufficiently high
temperature will begin to decompose into simpler substances, so this kind of reaction is common.
DOES A COMPREHENSIVE CLASSIFICATION
SCHEME EXIST ? If we searched we would find examples of other reactions that do not fit into these four categories. Indeed, it is probably fair to say that there is no
completely comprehensive classification scheme that
would accommodate all known chemical reactions. However, many, if not most, of the chemical reactions described
in your general chemistry text will fit into this simple
scheme. As you carry out each step of the cycle of copper
reactions, think about what is happening in each reaction,
and try to fit it into one of the four categories we described.
EXPERIMENTAL PROCEDURE
Special Supplies: Infrared lamps or steam baths, porcelain evaporating
dish.
Chemicals: 18- to 20-gauge copper wire, concentrated (16 M) HNO3, 3 M
NaOH, 6 M H2SO4, 30-mesh zinc metal, 6 M HCl, methanol.
!
S A F E T Y P R E C A U T I O N S : Concentrated nitric
acid, HNO3, is hazardous. It produces severe burns on the skin,
and the vapor is a lung irritant. When you handle it, you
should use a fume hood while wearing safety glasses (as
always) and rubber or polyvinyl chloride gloves. A polyethylene
74
A CYCLE OF COPPER REACTIONS
A waste container should also be provided for the
methanol used to dry the product in Step 5.
1. Cu TO Cu(NO3)2. Cut a length of pure copper wire
that weighs about 0.5 g (about a 10-cm length of 20gauge copper wire). If it is not bright and shiny, clean it
with steel wool, rinse it with water, and dry it with a tissue. Weigh it to the nearest milligram, recording the
weight in your laboratory book. Coil the wire into a flat
spiral, place it in the bottom of a 250-mL beaker, and—in
the fume hood—add 4.0 mL of concentrated (16 M)
nitric acid, HNO3. (If a fume hood is not available, use
the apparatus shown in Figure 5-1.)
Record in your notebook a description of what you see.
Swirl the solution around in the beaker until the copper
has completely dissolved. What is in the solution when the
reaction is complete? After the copper has dissolved, add
deionized water until the beaker is about half full. Steps 2
through 4 can be conducted at your lab bench.
2. Cu(NO3)2 TO Cu(OH)2. While stirring the solution with a glass rod, add 30 mL of 3.0 M NaOH to precipitate Cu(OH)2. What is formed in the solution besides
Cu(OH)2? Record your observations in your lab book.
3. Cu(OH)2 TO CuO. Stirring gently with a glass rod
to prevent “bumping” (a phenomenon caused by the formation of a large steam bubble in a locally overheated
area), heat the solution just barely to the boiling point
over a burner, using the apparatus shown in Figure 5-2. If
the solution bumps you may lose some CuO, so don’t
neglect the stirring. Record your observations. When the
transformation is complete, remove the burner, continue
FIGURE 5-1
If a fume hood is not available, substitute this apparatus.
squeeze pipet can be useful for transferring the HNO3 from a
small beaker to your 10-mL graduated cylinder. Rinse your
hands with tap water after handling HNO3. The dissolution of
the copper wire with concentrated HNO3 should be carried out
in a fume hood. If no hood is available, construct the apparatus
shown in Figure 5-1 to substitute for the fume hood. The
brown NO2 gas that is evolved is toxic and must be avoided.
NaOH solutions are corrosive to the skin and especially dangerous if splashed into the eyes—wear your safety glasses.
Methanol and acetone are flammable and their vapors
are toxic. Use them in the hood to avoid breathing the vapor,
and keep them away from all open flames.
W A S T E C O L L E C T I O N : The supernatant solution
that is decanted in Step 5 contains zinc sulfate and zinc chloride and should be collected.
CuO
suspension
Warm
gently,
only as
necessary
FIGURE 5-2
Setup for heating Cu(OH)2 to convert it to CuO.
A CYCLE OF COPPER REACTIONS
stirring for a minute or so, then allow the CuO to settle.
Then decant (pour off) the supernatant liquid, being
careful not to lose any CuO. Add about 200 mL of hot
deionized water, allow to settle again, and decant once
more. What is removed by this washing and decantation
process?
4. CuO TO CuSO4. Add 15 mL of 6.0 M H2SO4, while
stirring. Record your observations. What is in solution
now? Now transfer operations back to the fume hood.
5. CuSO4 TO Cu. In the fume hood, add all at once
2.0 g of 30-mesh zinc metal, stirring until the supernatant
liquid is colorless. What happens? What is the gas produced? When the evolution of gas has become very slow,
decant the supernatant liquid, and pour it into the waste
container provided. If you can see any silvery grains of
unreacted zinc, add 10 mL of 6 M HCl and warm, but do
not boil, the solution. When no hydrogen evolution can
be detected by eye, decant the supernatant liquid, and
transfer the copper to a porcelain dish. A spatula or rubber policeman is helpful for making the transfer. Wash
the product with about 5 mL of deionized water, allow it
to settle, and decant the wash water. Repeat the washing
and decantation at least two more times. Move to the
hood, away from all flames. Wash with about 5 mL of
methanol, allow to settle, and decant. Dispose of the
methanol in the proper receptacle. Place the porcelain dish
under an infrared lamp or on a steam bath or hot plate,
and dry the copper metal. What color is it? Using a spatula, transfer the dried copper metal to a preweighed
100-mL beaker and weigh to the nearest milligram. Calculate the mass of copper you recovered by subtracting
the weight of the empty beaker from the weight of the
beaker plus the copper metal.
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CALCULATION OF PERCENTAGE RECOVERY.
Express the percentage of copper recovered as
percentage
mass of recovered copper
recovery initial mass of copper wire 100%
If you are careful at every step, you will recover nearly 100
percent of the copper you started with.
☛
Consider This
If you used a penny as the source of your original copper
in this experiment, would it matter if you used a pre1982 penny (essentially pure copper) or a post-1982
penny (copper cladding over a zinc core)? Describe what
would happen in each step if you used a post-1982
penny. Test your predictions.
Why would it be hard to perform a cycle of oxygen or a
cycle of hydrogen experiment similar to this cycle of copper exercise? Can you design an experimental apparatus
for a cycle of oxygen lab?
BIBLIOGRAPHY
Bailar, Jr., J.C. “A Further Improvement on the Copper
Cycle Experiment,” J. Chem. Educ., 1983, 60: 583.
Condike, G. F. “Near 100% Yields With the ‘Cycle of Copper Reactions’ Experiment,” J. Chem. Educ., 1975, 52:
615.
Todd, D. and Hobey, W.D. “An Improvement in the Classical Copper Cycle Experiment,” J. Chem. Educ., 1985,
62: 177.
Umans, T. and de Vos, W. “An Improved Copper Cycle
Experiment,” J. Chem. Educ., 1982, 59: 52.
REPORT
A Cycle of Copper Reactions
5
NAME
SECTION
LOCKER
INSTRUCTOR
DATE
DATA AND CALCULATIONS
1. Initial mass of copper wire
g
2. Mass of beaker plus dry copper
g
3. Mass of empty beaker
g
4. Mass of recovered copper
g
5. Percentage recovery
percentage recovery mass of recovered copper
100% initial mass of copper wire
%
Show your calculation.
EQUATIONS AND OBSERVATIONS
For each step of the cycle, write the products of the reaction and balance the chemical equation(s). Using the
classification scheme presented in the Pre-Lab Preparation, write the reaction type (combination, decomposition, or
single/double replacement). Also record your observations of what happens at each step, and answer the questions
posed earlier in the Experimental Procedure.
1. Cu HNO3 O2 B
Reaction type:
What is in the solution after reaction is complete?
Observations (Be sure to include color and texture changes that occur in each step.)
77
2. Cu(NO3)2 NaOH B
Reaction type:
HNO3 NaOH B
Reaction type:
What is formed in the solution besides Cu(OH)2?
Observations:
heat
3. Cu(OH)2 ßßßßB
Reaction type:
What is removed by the washing and decantation process at the end of Step 3? (Consider the products of the
reaction as well as reagents from previous steps.)
Observations:
4. CuO H2SO4 B
Reaction type:
What is in the solution at the end of Step 4?
Observations:
78
REPORT
5
NAME
Sheet 2
5. Zn CuSO4 B
Reaction type:
Zn H2SO4 B
Reaction type:
Zn HCl B
Reaction type:
What happens when the zinc is added?
What is the gas produced in the reaction?
What is removed by the washing and decantation near the end of Step 5?
What color is the recovered copper?
Observations:
79
QUESTIONS
1. Describe whether the error introduced by each of the following problems would result in a high or a low value for
the Cu recovery, or would not affect the results.
(a) Some of the copper nitrate solution is splashed out of the beaker in Step 1.
(b) Insufficient NaOH is added in Step 2.
(c) The solution bumps in Step 3, splashing out some CuO.
(d) Some solid is lost in the decanting process.
(e) An excess of H2SO4 is added in Step 4.
(f) Some unreacted zinc remains with the product at the end of the experiment.
80
REPORT
5
NAME
Sheet 3
(g) The washings in Step 5 are insufficient to remove all of the solution residues from the copper.
(h) The copper is not completely dried.
2. How many milliliters of 3.0 M NaOH are required to react with 4.0 mL of 16 M HNO3?
3. How many milliliters of 3.0 M NaOH are required to react with 0.5 g of Cu2 to form Cu(OH)2?
4. (a) Add together the results you calculated for Questions 2 and 3 and compare the sum with the milliliters of 3.0 M
NaOH added in Step 2.
(b) Is there an excess of NaOH above that required to react with the HNO3 and Cu2?
(c) What would be the effect on the percentage recovery if not enough NaOH were added in Step 2 to react with
both the unreacted HNO3 and the Cu2 present at the end of Step 1?
81
☛
Consider This
If you used a penny as the source of your original copper in this experiment, would it matter if you used a pre-1982
penny (essentially pure copper) or a post-1982 penny (copper cladding over a zinc core)? Describe what would happen in each step if you used a post-1982 penny. Test your predictions.
Why would it be hard to perform a cycle of oxygen or a cycle of hydrogen experiment similar to this cycle of copper
exercise? Can you design an experimental apparatus for a cycle of oxygen lab?
82